This invention relates to instruments, particularly medical and research instruments that are used for assessing gas volumes of air cavities, particularly, air cavities that may exhibit a compliance to changes in pressure, such as in vivo volumes of the lung, thorax, oropharynx and/or nasopharynx.
Over the years, a number of methods have been used to determine the functional residual capacity (FRC) of the lung and related thoracic gas measures of a patient. These methods have involved gas dilution techniques, body plethysmography, and radiographic techniques. Gas dilution techniques require the patient to inhale special gases and necessitate special ventilation facilities (see, for example, U.S. Pat. No. 6,139,506). Radiographic techniques require a patient to be exposed to radiation. Additionally, static chest wall and abdominal composure by the patient is required during imaging. Plethysmography requires enclosing the patient or most of the patient""s body (see, for example, U.S. Pat. Nos. 5,513,648 and 5,159,935) in a sealed enclosure or at the very least outfitting the patient with impedance belts about the torso (see, for example, U.S. Pat. No. 5,857,459). For these methods, lung pressurization maneuvers are performed by the patient during which changes in lung volume are simultaneously assessed by the plethysmograph. The general gas equation, relating pressure and volume and changes in pressure and volume, is used to determine the unknown volume. Current plethysmographic techniques to assess thoracic gas volume suffer from artifacts due to stomach gas, which causes compliance during testing maneuvers.
A method to estimate xe2x80x9ctrappedxe2x80x9d air volume (not absolute volume) in lung of paralyzed patients has been proposed by obtaining a volume/pressure curve upon forced ventilation of the patient""s lung (see U.S. Pat. No. 4,844,085). The large volume of gas exchange with this method introduces errors that must be compensated and the forced pressurization/de-pressurization precludes normal breathing of the patient during testing. None of the above methods allow convenient isolation and measurement of the volume of the oral cavity and nasal pharynx.
U.S. Pat. No. 5,937,854 discloses a method and apparatus for ventilator pressure and optimization by administering fixed stepwise pressure changes to the lungs of a patient and measuring the lung volume change resulting from each pressure change. The lung volume change is measured by using the RIP technique. This utilizes two elastic cloth bands containing insulated wires, which encircle the patient""s rib cage and abdomen and are connected to an oscillator module.
It is accordingly a principal object of the present invention to provide a non-invasive device and method for measuring in vivo gas volumes of a patient, including lung and pharyngeal volumes and, particularly, to obtain volume measurement in the presence of compliance.
An additional object of the present invention is to provide an inexpensive device and method that measures the lung volume of a patient independent of a sealed chamber or ventilated airspace and that does not require outfitting the patient with respiratory bands.
A further object of the present invention is to provide a device and method to measure lung and airway volume of a patient by a means that is not dependent upon patient cooperation and participation. In other words, the patient is only required xe2x80x9cto breathexe2x80x9d and not to perform specialized pressurization maneuvers to within a certain tolerance. Therefore yet another object of the present invention is to provide a device for measuring the lung volume of the immobile, paralyzed, and xe2x80x9cintensive carexe2x80x9d or xe2x80x9cspecial carexe2x80x9d patient.
Accordingly, as will be disclosed in detail below, several advantages of the present invention are the measurement of in vivo volumes with a device that is smaller and more portable than existing systems, a device and method that is less complicated for clinicians and less troublesome for patients, and a device and method that serves a greater patient population, including veterinary applications, than is heretofore possible.
The purpose of the present invention is to provide a non-radiographic, noninvasive, portable, and non-confining apparatus for measuring gas volumes of in vivo cavities, including but not limited to lung volume and volumes of the thorax, oral and nasal pharynx. Further, the apparatus does not require sophisticated lung pressurization maneuvers to be performed by the patient or the outfitting of patients with thoracic position transducers. The present invention is intended therefore to serve a comprehensive patient population, including the bedridden, unawake, paralyzed, and sedated patient. Further, the device does not require the patient to inhale special gases or be subjected to imaging radiation.
It is recognized that various methods exist for assessing lung volume. The present invention represents improvements in the apparatus of boxless measurement of lung volume that can take the form of several embodiments. The detailed embodiments described herein are taken as representative or exemplary of those in which the improvements of the invention may be incorporated and are not presented as being limited in any manner.
The invention is directed to an apparatus for measuring gas volumes of an in vivo cavity of unknown compliance in a subject, particularly a patient comprising:
(a) an air cavity with induction means for inducing calibrated volume changes in said air cavity;
(b) a means for interfacing said air cavity to the in vivo volume of the subject to be measured;
(c) a means connected to said air cavity for measuring air pressure variations; and
(d) a control means electrically coupled to said induction means and measuring and processing means for calculating the gas volume in said subject.
In one embodiment, the subject is a human patient; in another embodiment the subject is a mammal; in yet another embodiment, the subject is a non-living item with a cavity exhibiting compliance, such as a balloon, a tank containing a bladder, or a tank with an inverted floating cover such as one used to contain hydrogen or natural gas.
The apparatus interfaces an air cavity to particularly the patient by means of a facial mask, nasal mask, mouthpiece or tubes. In a preferred embodiment, the interfacing means is a facial mask so that a common air cavity is formed with the patient via the oral and/or nasal orifices. The apparatus includes a respiratory access valve connected to its inner cavity that, when open, permits the patient to exchange air with the external environment in the manner of ordinary breathing (means for interfacing said air cavity to ambient environment) and, when closed, permits artificial pressurization of the cavity by means of a calibrated volume-changing piston (means for inducing volume change). The apparatus includes a calibrated device to assess air pressure changes occurring inside the common air cavity and a device to assess air pressure of the ambient environment.
The valve interfaces between the external environment and the inner cavity of the apparatus, and is opened or closed by passive means according to breathing airway pressure of the patient. The valve is constructed in such a manner as to remain open while the patient is in the process of inhaling or exhaling, and to momentarily close during the period of time that the patient is changing breathing modality from exhalation to inhalation, when cavity pressure is beneath the shutter threshold. The pressure change in the system due to the induced change in volume is, in itself, insufficient to open the valve.
The invention is also directed to a method for measuring a gas volume of an in vivo cavity in a subject utilizing the apparatus of the present invention comprising
(a) attaching said apparatus to said subject;
(b) measuring the barometric pressure in an area near the subject;
(c) measuring changes in induced pressure and volume in said cavity during an induction and preferably at least two inductions, and
(d) calculating said gas volume.
The method may further comprise the step of calculating compliance of said cavity where compliance is present.
The control and processing unit monitors system pressure during the breathing cycle and is therefore programmed to determine if pressure is negative (indicating inhalation), positive (exhalation), or zero (peak of inhalation or trough of exhalation). When the processing and control unit assess that air cavity pressure is within the range that the respiratory valve has become closed and, further, that the breathing cycle is at the trough of exhalation, the control unit repositions the piston and thereby decreases the system volume by a small, known amount. Those versed in the art comprehend that pressure and volume of gas in a closed mass system are mathematically related by the general gas equation. Specifically, the pressure-volume product of the system gas prior to piston movement is equal to the pressure-volume product after piston movement, for the same gas temperature.
Compliance occurs whenever the volume under test changes as a result of increased inner forces due to pressurization of the air cavity. Possible sources of compliance are cheeks and lung wall. Of particular note is compliance of the lung wall due to stomach gas, which is an artifact of body plethysmography of all types. Those versed in the art will appreciate the difficulty of measuring a volume under the circumstances in which that volume might adjust itself to the increased pressure created by the measurement process. The invention proposes a means of determining compliance in the course of testing by applying a plurality of different induced gas volumes that result in different pressure measurements from which in vivo volume may be determined.